Method of indirect steam generation



March 1969 R. STROEHLEN 3,430,608

METHOD OF INDIRECT STEAM GENERATION Filed Feb. 28, 1967 Sheet of 4 j PRIOR AU Lb'ffter Boiler- INVENTOR Richard Sfrvehlen ATTORN J March 1969 R. STROEHLEN 3,430,608

METHOD OF INDIRECT STEAM GENERATION Filed Feb. 28, 1967 Sheet 2 dr 4 HEAT EXCHANGER HEIIT EXCHANGE Z INVENTOR k (m Fat/70rd Szroeh/en BY v 4,44% M %EYJ March 4, 1969 R. STROEHLEN 3,430,608

METHOD OF INDIRECT STEAM GENERATION Filed Feb. 28, 1967 Sheet 3 of 4 Fz' 4 "771 Q 9- I l m T(m +mm m 10 5 72 a o HEAT EXCHANGER mT \1 INVENTOR Richard Szroehlen W MMATTORNEYJ March 4 1969 R. STROEHLEN 3 430 0 METHOD OF INDIRECT STEAM GENERATION Filed Feb. 28. 1967 Sheet 4 of 4 HEAT EXCHANGER 1 Ff'yffi INVENTOR Kz'chard Szroehlen ATTORNEYS United States Patent 3,430,608 METHOD OF INDIRECT STEAM GENERATION Richard Strochlen, Nuremberg, Germany, assignor to Maschinenfabrilr Augsburg-Nuremberg, Akfiengesellschaft, Nuremberg, Germany Filed Feb. 28, 1967, Ser. No. 619,257 Claims priority, application Germany, Mar. 2, 1966,

M 68,39 U.S. Cl. 122--3l Int. Cl. FZZb 1/12 5 Claims ABSTRACT OF THE DISCLOSURE A method of indirect steam generation is known where the supply of the actual heat of evaporation to produce high-pressure superheated steam from highly preheated feed water is effected not by direct transmission of heat from flue gases to the evaporating water in the boiler, but where the boiler serves exclusively to heat saturated steamat a rate constituting a predetermined multiple of the useful steam flow to be producedto the desired superheated steam temperature, with the steam flow exceeding the useful steam flow being used as heating steam to heat the feed water to boiling temperature, and to evaporate the feed water, in a steam drum located outside the boiler. In this method, known as the Loffler system, the steam drum operates as a direct-contact evaporator in which the heating steam gives off its superheat and leaves the drum together with the saturated steam raised from the feed water. This so-called circulating steam flow is supercharged in a compressor by a pressure differential which is equal to the pressure losses in the boiler and the pipe system. In other words, there is only one uniform medium, namely steam, to be heated in the boiler whereby difliculties encountered with twophase mixtures varying in steam quality from O to 100% as in the normal direct steam raising methods are inherently avoided.

In the case of conventional design forced-circulation boilers, it is extremely difficult to have equal flows in all heating surface tubes connected in parallel. Such unequal distribution tends to result in inhomogeneous steam/water mixtures of varying composition at the outlet of the individual heating surface sections and, consequently, in difficulties to establish stable flow. This, in principle, is eliminated in the systems of indirect evaporation where no stability difficulties exist and definite and predictable conditions exist in the steam generator or nuclear reactor respectively,

A factor of special importance is, furthermore, that the flow rate through the heat-absorbing heating surfaces is a multiple of that in conventional forced-circulation boilers. In this fashion, a greater multiplicity of parallelconnected tubes is assured of a positive supply of coolant and, thereby, excessive local heating is positively obviated.

A drawback of indirect steam generation of the abovedescribed method is in the fact that the necessary supercharging of the circulating steam flow involves additional power and that the associated saturated steam compresice sor constitutes an additional potential source of operating troubles.

The additional power required naturally affects the energy balance considerably and results in high auxiliary power requirements of such systems and, consequently, a tangible reduction of the thermal overall efficiency of power generation.

The task on which the present invention is based is to reduce to a fraction the compressor power required for steam generation according to the Lofiler principle, to obviate the need for extra power entirely in certain cases, or even to produce extra power.

Now, according to the solution offered by this invention, it is proposed, by means of superheated steam m to produce in a heat exchanger from one part m of the feed water flow m steam at such a pressure as is required at the inlet into the superheater, and to pass this steam directly into the superheater. In this manner, the compressor power otherwise required can be eliminated or reduced.

Furthermore, it is proposed to supercharge the heating steam flow after 'it has cooled down in the heat exchanger by means of a compressor by the amount of pressor loss in the system and, subsequently, to feed it to the direct-contact evaporator from which the saturated steam produced therein flows directly, i.e. without a compressor, into the superheater.

According to a further feature of this invention, the heat exchanger may take the form of a surface evaporator or a counterflow heat exchanger. Likewise, it is possible to connect the heat exchangers in series at the heating steam side.

Moreover, it is proposed by this invention to have part of the feed water, which is not directly fed to the direct-contact evaporator, divided into several partial flows having different pressures which are each passed through a separate heat exchanger.

Also covered by this invention is a system where, with the exception of the steam produced in the final pressure stage, the steam produced in each pressure stage is made to drive a turbine from which it is exhausted at the pressure required ahead of the superheater. It is also proposed in this invention that partial flows of the feed water of various pressure stages may be supplied from a common feed pump.

Another feature proposed by this invention is a system where one or several partial flows of the feed water are supplied, after being discharged from the feed water pump, via feed heaters to the heat exchangers, i.e. a surface evaporator and a counterfiow heat exchanger or direct-contact evaporator respectively. Finally, it is proposed by this invention that the turbines of the various pressure stages may be coupled in tandem with the compressor.

The means by which the objects of this invention are obtained are described more fully with reference to the accompanying drawings which show several examples of both the prior art and the method according to the present invention in schematic form, and in which:

FIG. 1 is a circuit diagram of the conventional prior art Lofiler boiler;

FIGS. 2 and 3 each show, respectively, a simple circuit diagram of this invention; and

FIGS. 4 and 5 show, respectively, two further embodiments of the methods covered by this invention.

In the Lofiler boiler shown schematically in FIG. 1 which represents the prior art, the feed water flow m admitted through pipe 1 is, ignoring blowdown amounts, equal to the useful steam in discharged in pipe 2 for any consumer installations and is caused to evaporate in a direct-contact evaporator 3, designed in the form of a drum, by the heating steam flow m admitted to the latter through the pipe 4; the steam so produced is supercharged by the steam compressor 5, the amount of pressure rise being equivalent to the pressure loss occurring through the whole system. The heating steam supplied to the direct-contact evaporator 3 gives oft its superheat in it and leaves it also as saturated steam. In other words, the boiler proper 6, in which the primary energy is released in the form of heat, is supplied with the sum of the steam flow "1 and the useful steam flow m which is superheated there from the state at the outlet of the steam compressor-somewhat above the saturated steam stateto the desired superheat temperature of the useful steam. Downstream of the boiler 6, the steam fiow is divided into the actual useful steam flow m and the heating steam flow m fed into the direct-contact evaporator 3. An electric motor 7 is provided to drive the steam compressor 5.

For an evaluation of this conventional steam generating method there are two criteria which are of considerable importance, viz:

(a) The circulating ratio n which is the ratio of the steam flow passing through the steam compressor (nut-m to the useful steam flow m. The circulating ratio u therefore indicates what multiple of the useful steam flow m has to be supercharged by the compressor and flows through the boiler proper.

(b) The specific power for the drive of the compressor by the electric motor 7, i.e. a measure for the work (kwh.) that has to be expended to produce one ton of useful steam.

Simple power considerations on the basis of the energy balances show that both the circulating ratio u and the specific power for generating the useful steam in kwh./ ton are reduced to a minimum if the feed water temperature ahead of the direct-contact evaporator drum is equal to the saturation temperature in the direct-contact evaporator drum.

For the optimum case of preheating the feed water to saturation temperature, e.g. by multi-stage regenerative feed heating, the characteristic values for the Lofiler boiler, assuming a pressure loss in the system of 10%; a compressor efiiciency of 80%; and a motor efficiency of 95% are as follows:

KgL/cm. one atmosphere.

As the heat of evaporation of steam is substantially reduced as the critical pressure is approached, the necessary heating steam flow decreases in the same proportion and, consequently, also the specific power.

In FIGS. 2 to showing this invention, the numeral 1 again indicates the feed water supply pipe, 2 the pipe for the useful steam flow In, 3 the direct-contact evaporator, 4 the supply pipe for the heating steam flow m 5 the compressor, 6 the superheater, and 7 the electric motor for the drive of the compressor. The superheater 6 may also take the form of a nuclear reactor.

The numeral 8 designates a heat exchanger, or one of the heat exchangers, whereas 9 indicates a pressure-re ducing valve. The feed water flow In is divided into the quantity m which is evaporated in the heat exchanger 8 and into the remaining feed water flow (mm which is supplied via the pressure-reducing valve 9 to the directcontact evaporator 3 to be evaporated there. In other words, the feed water flow m taken to the direct-contact evaporator 3 is reduced by the amount m which is delivered to the heat exchanger 8. This partial flow m is available at a higher pressure so that its pressure need not be increased which is an essential criterion of this invention.

Furthermore, the steam flow (m-m +m in the compressor 5 which is driven by the electric motor 7 is supercharged to the pressure at the admission of the steam flow (m-J-m into the superheater or nuclear reactor 6 (FIG. 2). It is also within the scope of the present invention to supercharge the substantially smaller heating steam flow m to the necessary initial pressure ahead of the superheater or nuclear reactor 6 whereby the throttling of the feed water flow (mm is eliminated.

According to the circuit shown in FIG. 3, the heating steam "1 after cooling down in the surface heat exchanger 8 is supercharged in the compressor 5 and, subsequently, conveyed to the direct-contact evaporator 3.

FIGS. 4 and 5 show a few practical embodiments.

In FIG. 4, steam supplied at 490 C. via the pipe 4 is cooled in a counterfiow heat exchanger 11 to about 4l0 C. The counterfiow heat exchanger has a feed water partial fiow 111 admitted to it which is supercharged to 350 kgf./cm. i.e. to supercritical pressure, by a feed pump 16 and heated to 450. The partial flow m in the present case 33.7% of the feed water flow, is expanded from 350 kgf./cm. to 166 kgf./cm. and made t perform work in a turbine 12 and is delivered at this pressure to the superheater or nuclear reactor 6. Shaft 10 connects turbine 12 to compressor 5.

A further feed water partial flow m which is supercharged to the pressure at the inlet to the superheater 6, in this case to 166 kgf./cm. by the feed pump 16, is taken to, and evaporated in, the heat exchanger 8. The heating steam flow m is cooled from about 410 C. to 363 C. in the process, supercharged to 166 kgf./cm. in the compressor 5, and, eventually, led to the directcontact evaporator 3 in which the balance feed water fiow (mm -m is evaporated. Assuming a very conservative efiiciency of the turbine 12 of 50% and a moderate efiiciency of the compressor 5 of there will be an additional power demand for the turbo-set which has to be met by the electric motor 7. With a motor efiiciency of 96%, the specific power required is as low as 7.15 kwh./ton useful steam compared with 18.4 kwh./ton in the case of the conventional Lofiler boiler. The power savings are therefore quite substantial and would be even higher with better machine efficiencies. The circulating ratio is 11:2.48.

The feed water heaters 13, 14 and 15 in FIG. 4 serve to preheat the feed water partial flows preferably to boiling temperature and they can be integrated structurally and flow-wise into a single unit.

The various feed water partial flows may be supercharged to the necessary pressure in the common feed pump 16, it being possible, by an additional group of stages, to supercharge the partial flow m to supercritical pressure.

FIG. 5 shows a further circuit arrangement where no power at all is required for the generation of useful steam. Further essential elements provided are the pressure booster feed pump 17 and the tandem-coupled turbine 18. Compensation in the energy balance for the power consumed by turbine 12, the pressure booster feed pump 17, and the compressor 5 is effected by the tandemcoupled turbine 18 which is operated by the useful steam flow m. In this case, the pressure and temperature at the outlet of the superheater or nuclear reactor 6 is proposed to be rated so that the specified live steam condition is obtained :at the outlet from the turbine 18. Motor 7 is not used except under special circumstances.

Having now described the means by which the objects of the invention are obtained, I claim:

1. In a method of indirect steam generation comprising introducing feed water and heating steam into a directcontact evaporator to produce saturated steam, cornpressing the saturated steam, superheating the compressed saturated steam, dividing the superheated steam into useful steam and heating steam, and then sending the heating steam to the evaporator, the improvement further comprising heating a partial portion of the feed water with said heating steam in a heat exchanger locate d between the superheater and the evaporator to produce feed water steam at a pressure needed for entry into the superheater, and then introducing this feed water steam into the superheater.

2. In a method as in claim 1, further comprising a second counterflow heat exchanger ('11) joined to the first heat exchanger.

3. In a method as in claim 2, said first heat exchanger being joined in series to the second heat exchanger.

4. In a method as in claim 3, further comprising compressing said heating steam in a compressor (5) driven by a shaft common to a steam driven turbine (12).

5. In a method as in claim 1, further comprising using the useful steam to drive a turbine coupled to a pressure booster feed pump (17), a compressor (5) and a steam driven turbine (12) to balance the power required for turbine (12).

References Cited UNITED STATES PATENTS CHARLES J. MYHRE, Primary Examiner. 

